Capt. Larry Nodiff said the gun was a .40-caliber Smith & Wesson semiautomatic pistol with one live round in the chamber and eight live rounds in the magazine. Jones did not have a permit to carry the weapon, Nodiff said.
The serial number on the gun was obliterated, he said, but police were using forensic techniques to try to recover it. He said there was no indication that Jones intended to use the gun at the school.
Principal Otis Hackney said the incident was over in minutes. The school did not initiate a lockdown.
"There was no sense of an immediate threat," Hackney said. "All of the systems we had in place worked."
Because of the split schedule demanded by state Keystone exams, which are being given for the first time, some students had arrived at 8 a.m. and others were due at 11. The building was half-empty when Jones arrived with the gun.
Also Thursday, a ninth-grade student was arrested when he tried to sneak a handgun into Benjamin Franklin High School, 550 N. Broad St., Gallard said.
The boy was trying to pass through a school metal detector about 12:50 p.m. when the gun was discovered hidden in one of his shoes, Gallard said. The gun was not loaded and was found to be inoperable.
School police arrested the boy and transferred him into city police custody.
The weapons incident at South Philadelphia High was the third this academic year. Between September and December, weapons were found twice at the school, according to district reports. The first weapon was classified as a "cutting instrument"; the second was listed as "other type."
In addition, between the 2005-06 school year and the end of 2011-12, there were 85 weapons incidents at the school and no fewer than six incidents in any year.
The number of weapons incidents per year, however, has been declining: There were 22 incidents at the school in 2010-11, nine in 2011-12.
Jones was with her infant and an aunt at the school Thursday morning, Nodiff said, and the child is now in the aunt's custody.
Jones has been charged with possession of a firearm with an altered serial number, carrying a gun without a license, carrying a weapon inside a school, and carrying a loaded weapon.
Contact Chris Palmer at 215-854-4567 or email@example.com, or follow on Twitter @cs_palmer.
Inquirer staff writers Dylan Purcell and Robert Moran contributed to this article.